Cellular Ceramics / 5.10
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Fig. 7 View of the PM engine head with open chamber built on the basis of one-cylinder DI diesel engine
self-ignition of the resulting mixture follows in the PM volume together with a volumetric combustion characterized by a homogeneous temperature distribution in the PM combustion chamber (Fig. 6d). During the following expansion stroke the heat is transformed into mechanical work (Fig. 6e). Again, all necessary conditions for homogeneous combustion are fulfilled in the PM combustion volume. To describe the main thermodynamic properties of the PM engine cycle, a thermodynamic model was proposed, and a more detailed thermodynamic analysis of the PM engine concept can be found in Ref. [9]. An example of a PM engine head with open chamber and PM reactor mounted in the engine head is shown in Fig. 7. The porousmedium reactor made of SiC is mounted in the head of a single-cylinder direct injection diesel engine. The injection nozzle is positioned inside the porous medium reactor and permits very good homogenization throughout the PM volume. The reactor volume is approximately 45 cm3 with a mean pore diameter of 3 mm. The outer surface of reactor consists of solid wall with thermal inslation.
5.10.5
An Update of the MDI Engine Concept: Intelligent Engine Concept with PM Chamber for Mixture Formation
The MDI (mixture direct injection) concept covers the mixture-formation and heatrecuperation system. This concept offers homogenization of the combustion process by performing fuel vaporization, its chemical recombination (low-temperature oxidation processes, e.g., cool and blue flames), and energy recirculation in a porous-
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medium chamber. The enthalpy of the burned gases is partly transferred to the porous medium and can later be supplied back to the cylinder. This energy is utilized for both vaporization of liquid fuel and for its chemical recombination in the PM volume [7, 20, 21].
A practical approach to the MDI system requires a porous-medium chamber to be mounted in proximity to the cylinder and equipped with a valve allowing contact between PM chamber and cylinder volume. The engine cycle described below models the real engine cycle, and timings for the PM chamber other than those presented here can be used. The MDI concept can be combined with conventional combustion modes, such as GDI (gasoline direct injection), HCCI (homogeneouscharge compression ignition), and with radical combustion (RC), and only control of the PM-chamber timing is necessary to select a combustion mode in the engine [7]. By applying the variable timing of the PM chamber, the MDI concept offers a
Fig. 8 Characteristic phases of MDI system operation, here illustrated as an updated concept in association with a porous medium in the engine head.
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combination of individual combustion modes in one engine, as described below. Characteristic phases of the cycle with MDI mixture preparation are illustrated in Fig. 8. In phase I the PM chamber is charged with burned gases containing energy (Fig. 8a). In phase II the liquid fuel is injected into the PM chamber and fuel vaporization occurs (Fig. 8b). In phase III gaseous charge containing evaporated fuel, energy, and active radicals discharges from the PM chamber into the cylinder (noncombustible mixture; Fig. 8d), and in phase IV mixing with air in the cylinder and ignition of combustible mixture take place (Fig. 8e). The system considered in Fig. 7 consists of the cylinder with a moving piston and the PM chamber equipped with a poppet valve. This valve allows control of the PM-chamber timing. A detailed analysis of the MDI concept can be found in Ref. [7].
Combination of the MDI concept with other individual conventional combustion systems (GDI, HCCI, RC) provides several advantages, including extension of lean effective limit and improved ignitability of a homogeneous charge, reduction of temperature peaks under lean operation conditions (homogeneous charge), elimination of soot, better and faster homogenization of the charge in the cylinder, and control of the concentration of active radicals almost independent of the cylinder conditions.
For homogeneous combustion under variable engine load and speed, ignition conditions and charge reactivity are also required to be variable. This variability means variable ignition and combustion mode. The combination of these variable conditions allows not only realization of homogeneous combustion conditions but also control of ignition timing and heat-release rate. Both aspects define the practicability of the combustion system operating under homogeneous combustion conditions. Thus, the variable timing of the MDI concept allows control of the cylinder charge parameters, which is necessary for realization of homogenous combustion: TDC compression temperature; temperature history during the compression stroke; reactivity of the charge; homogeneity of the charge; and heat capacity of the charge. This variability defines the “intelligence” of the engine.
5.10.6
Two-Stage Combustion System for DI Diesel Engine
Another application of porous-medium technology to mixture formation and combustion in the DI diesel engine is the two-stage combustion concept. This concept offers control of mixture formation and combustion conditions in direct injection diesel engines by spatial splitting of the combustion process into three zones and two time phases, as shown in Fig. 9. Zone 1 is the volume of the inner part of the PM ring, zone 2 the volume in the PM ring itself, and zone 3 the free volume between outer part of the PM ring and the piston bowl.
Main features of the two-stage combustion system are:
The system operates under two characteristic conditions related to partial and full load (i.e., small amount of fuel injected under low pressure, and large amount of fuel injected under high pressure).
5.10 Porous Media in Internal Combustion Engines 593
|
Injector |
Zone 3 |
|
Zone 1 |
Zone 2 |
|
PM-Ring |
Piston |
|
Fig. 9 Principle of PM ring in the piston bowl of a diesel engine for two-stage combustion system.
The PM divides the combustion chamber into three parts (zones 1 to 3) and significantly influences the fuel distribution, fuel vaporization, mixing with air, and generates turbulence during the gas flow through the PM ring.
The PM only takes part in the combustion process, but it significantly influences the temperature of the zone by accumulating part of the energy released, and improves self-ignition from the hot PM walls.
Generally, the system operates in such a way that the following mixture conditions are achievable: rich mixture (in a free volume), lean mixture (in a free volume), and any mixture composition in PM. Combustion of such a mixture permits low NOx and low soot emissions.
Generally, two stages of the combustion process can be selected, independent of the operational conditions: early stage, from late compression until TDC (upward piston motion); late stage, from TDC until completion of combustion (downward piston motion).
Transition between the two stages of combustion is connected with a strong gas flow through the PM generating turbulence and significantly improving the mixing of gases. In both stages of combustion the porous medium (partly) controls the ignition and combustion process
Generally, the application of porous-medium technology to gas flow processes, reduces engine dependence on large-scale in-cylinder flow structures (e.g., swirl, tumble) and the turbulence generated in the cylinder. If the gas is pushed into or through the PM volume, strong heat transfer from the solid phase of the PM and gas is observed together with a spatial homogenization of the gas in the PM volume. One of most critical aspects in conventional combustion systems is injection of liquid fuel. If the liquid fuel is injected directly into the PM volume, fuel atomization and spray geometry are not critical. A self-homogenization process in the PM volume is observed and leads to spatial distribution of the liquid fuel throughout the
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PM volume (Fig. 3). There are four characteristic phases of jet interaction with the porous medium [8]: phase A representing exiting from the nozzle and free jet formation; phase B representing jet interaction with the PM interface; phase C representing liquid distribution throughout the PM volume; and phase D representing liquid leaving the PM volume. Mixture formation and charge homogenization conditions in conventional engines are very complex and difficult to control. Significantly different conditions of mixture formation occur in porous media, especially where a 3D porous medium structure controls charge homogenization and fuel distribution in the PM volume. In this case, the conditions of mixture formation are almost independent of the engine operational conditions. The art of ignition and resulting combustion process in conventional engines depends on the fuel and injection conditions.
In a porous-medium reactor (e.g., PM engine system), independent of the engine operational conditions, 3D thermal PM self-ignition of the homogeneous charge is realized in the PM volume. The combustion process is characterized by a homogeneous and controlled temperature in the whole PM chamber volume, and no combustion occurs in the free volume of the cylinder. The maximum temperature is reduced by heat accumulation in the porous medium, giving rise to ultralow NOx emissions independent of the engine operational conditions.
5.10.7
Summary
In this chapter novel concepts for combustion engines in diesel-fuelled vehicles were discussed. It was shown that porous media, especially those with a foam structure, can be used for a great variety of improvements in the combustion process. The key factor for NOx abatement and elimination of soot emission is a homogeneous combustion of the air/fuel charge in the cylinder volume. This can be realized by homogeneous mixture formation, prevention of formation of a flame front having a temperature gradient, and 3D ignition in the entire combustion volume. All these processes can be controlled or influenced with the help of porous media/ceramic foams.
References
1Kusaka, J., Yamamato, T., Daisho, Y., Simulating the Homogeneous Charge Compression Ignition Process Using a Detailed Kinetic Model for n-Heptane Mixtures, Int. J. Engine Res., 2000, 1 (3) 281–289.
2Urushihara, T., Hiraya, K., Kakuhou, A., Itoh, T., Parametric Study of Gasoline HCCI with Various Compression Ratios, Intake Pressures and Temperatures, Proc. A New
Generation of Engine Combustion Processes for the Future?, Ed. Duret, P., 2001, pp. 77–84.
3Coma, G., Gastaldi, P., Hardy, J.P., Maroteaux, D., HCCI Combustion: Dream or Reality? Proc. 13th Aachen Colloquium on Vehicle and Engine Technology, 2004, Aachen, pp. 513–524.
4Aceves, S.M., Flowers, D.L., Martinez-Frias, J., Smith, J.R., Dibble, R., Au, M., Girard, J.,
HCCI Combustion: Analysis and Experiments, SAE Technical Paper
No. 2001-01-2077, 2001.
5Montorsi, L., Mauss, F., Bhave, A., Kraft, M., Analysis of a Turbocharged HCCI Engine using a Detailed Kinetic Mechanism,
2002 European GT-SUITE Users Conference, October 21, 2002, Frankfurt, Germany.
6Weclas, M., New Strategies for Homogeneous Combustion in I.C. Engines Based on the Porous Medium (PM)-Technology, ILASS Europe, June 2001.
7Weclas, M., Strategy for Intelligent Internal Combustion Engine with Homogeneous Combustion in Cylinder, Sonderdruck Schriftenreihe University of Applied Sciences in
Nuernberg, 2004, No. 26, pp. 1–14.
8Weclas, M., Ates, B., Vlachovic, V.,
Basic Aspects of Interaction Between a High Velocity Diesel Jet and a Highly Porous Medium (PM), 9th Int. Conference on Liquid Atomization and Spray Systems ICLASS, 2003.
9Durst, F., Weclas, M., A New Type of Internal Combustion Engine Based on the Porous-Me- dium Combustion Technique, J. Automobile Eng. IMechE D, 2001, 215, 63–81.
10Durst, F., Weclas, M., A New Concept of IC Engine with Homogeneous Combustion in Porous Medium (PM), 5th International Symposium on Diagnostics and Modelling of Combustion in Internal Combustion Engines, COMODIA, 2001, Nagoya, Japan, Paper No. 2-27, pp. 467–472.
11Park, C-W., Koviany, M., Evaporation-Com- bustion Affected by In-Cylinder, Reciprocating Porous Regenerator, Trans. ASME, 2002, 124, 184–194.
12Hanamura, K., Nishio, S., A Feasibility Study of Reciprocating-Flow Super-Adiabatic Combustion Engine, The 6th ASME-JSME Thermal Engineering Joint Conference, 2003, Paper No. TED-AJ03-547.
13Leissner, H.F., US Pat. 1,260,408, 1918.
14Pfefferle, W.C., US Pat. 3,923,011, 1972.
15Bernecker, G., US Pat. 4,103,658, 1978.
16Firey, J.C., US Pat. 4,381,745, 1983.
17Siewert, R.M., US Pat. 4,480,613, 1984.
18Ferrenberg, A.J., US Pat. No. 4,790,284, 1988.
19Durst, F., Weclas, M., German Pat. 197 53 407, US Pat. 6,125,815, 1997.
5.10 Porous Media in Internal Combustion Engines 595
20Weclas, M., German Pat. Appl. 198 13 891,
1998.
21Weclas, M, German Pat. Appl. 101 35 062.7,
2001.
22Durst, F., Weclas, M., M.ßbauer, S., A New Concept of Porous Medium Combustion in I.C. Engines, International Symposium on Recent Trends in Heat and Mass Transfer, 2002, Guwahati, India.
23Mayer, A., Definition, Measurement and Filtration of Ultrafine Solid Particles Emitted by Diesel Engines, ATW-EMPA-Symposium,
19 April 2002.
24Particulate Traps for Heavy Duty Vehicles, Report of Swiss Agency for the Environment, Forests and Landscape (SAEFL), 2000, Environmental Documentation No. 130.
25Emission Control Retrofit of Diesel-Fueled Vehicles, Report of Manufacturers of Emission Controls Association, March 2000.
26Bovonsombat, P., Kang, B-S., Spurk, P., Klein, H., Ostgathe, K., Development of Current and Future Diesel After Treatment Systems, MECA/AECC Meeting, Bangkok, February 1, 2001.
27Sch fer-Sindlinger, A., Vogt, C.D., Hashimoto, S. Hamanaka, T., Matsubara, R., New Materials for Particulate Filters in Passenger Cars, Auto Technol., 2003, No. 5.
28Jacob, E., D Alfonso, N., Doering, A., Reisch, S., Rothe, D., Brueck, R., Treiber, P., PM-KAT: a Non-Blocking Solution to Reduce Carbon Particle Emissions of EuroIV Engines, 2002, 23. Internationales Wiener
Motorensymposium, 25–26 April 2002, Vol. 2: VDI Reihe 12, Nr. 490, VDI Verlag, Duesseldorf, 2002, pp. 196–216.
29Weclas, M., Melling, A., Durst, F., Unsteady Intake Valve Gap Flows, SAE Technical Paper, No. 952477, 1995.
30Weclas, M., Melling, A., Durst, F., Flow Separation in the Inlet Valve Gap of Piston Engines, Progr. Energy Combust. Sci., 1998, 24
(3), 165–195.
31Brenn, G., Durst, F., Trimis, D., Weclas, M., Methods and Tools for Advanced Fuel Spray Production and Investigation, Atomiz. Sprays, 1997, 7, 43–75.